Can anyone tell me the easiest way to replace the PCV valve on a 1999 a4 quatro 1.8t? I don't see it anywhere.

 

it's on the turbo inlet pipe just south of the airbox. This picture is sort of an odd angle, but its the best one i could find. The corrugated bit you see at the bottom is the turbo inlet pipe. The thing to the right of the red circle is the PCV valve. Above the circle, you can see part of the exhaust mani.



If you're going to the trouble, you might think about replacing it with a catch can. This DIY is for the MkI TT, which has a transversely mounted 1.8T, but the same general principle applies to yours. Hope that helps!